Sensor chips wherein a pair of temperature-sensitive elements Ru and Rd are formed with a heater element Rh therebetween, on a semiconductor substrate (for example, a silicon substrate) B, as illustrated, for example, in FIG. 8, have been the center of focus as a flow rate sensor capable of detecting with high precision fluid mass flows of a variety of fluids. Note that the pair of temperature-sensitive elements Ru and Rd and the heater element Rh are each formed from thin-film resistors of, for example, platinum (Pt). The heater element Rh and the pair of temperature-sensitive elements Ru and Rd are provided lined up along the direction of flow F of the fluid on a thin diaphragm D that is provided spanning a cavity (the recessed portion) C that is formed on the semiconductor substrate B, as illustrated in FIG. 9, which illustrates the cross-sectional structure thereof schematically. Note that in the figure, Rr is a temperature-detecting element for detecting the air temperature (fluid temperature) in the ambient environment of the sensor chip, provided at a location that is separate from the diaphragm D on the semiconductor substrate B.
The thermal flow meter that uses this type of sensor chip, as illustrated in, for example, Japanese Unexamined Patent Application Publication 2003-247876, focuses on the difference in the temperatures detected by the pair of temperature-sensitive elements Ru and Rd, produced by converting, into a flow rate (flow speed) Q of the fluid, the temperature distribution in the vicinity of the diaphragm D that results when the ambient temperature in the vicinity of the temperature-sensitive elements Ru and Rd, and, in particular, the heating temperature of the heater element Rh, is increased to a particular temperature higher than the ambient temperature (the fluid temperature) that is detected by the temperature detecting element Rr. The structure is such that the mass flow rate Q of the fluid can be calculated from the difference in temperatures detected by the pair of temperature-measuring elements Ru and Rd.
Even though the thermal flow meter with a structure set forth above is small and has high accuracy, it has problems in securing a broad range for the flow rate measurement range (the dynamic range). Because of this, conventionally a broad flow rate measurement range (dynamic range) has been secured through, for example, using a thermal flow meter for low flow rate measurements in parallel with a thermal flow meter for high flow rate measurements. However, typically it is difficult to assemble together a plurality of sensor chips (flow rate sensors) having different flow rate measurement ranges in a narrow flow path for the flow rate measurement, branched from a main flow path made from the fluid flow tube. Furthermore, when a plurality of flow sensor chips having different measurement flow rate ranges is assembled together in this type of narrow flow path, there is the risk of causing an interference with the measurement of the low flow rate.